One common form of an industrial fuse comprises a cylindrical housing having an axial central passage therethrough divided by partitions into three compartments or chambers. Disposed within the outer compartments are axially spaced short circuit blowing fuse elements, interconnected by a central slow blowing fuse element located in the central compartment. The slow blowing fuse element is generally made of low temperature melting alloy, such as solder, which melts well below 1000.degree. F. unlike, for example, copper alloy fuse elements. The slow blowing fuse referred to is one which will blow because the fuse element material itself reaches a melting point where it melts, collapses and opens the circuit. This type of slow blowing fuse, to be referred to as a center melting fuse, is to be contrasted with a diffusion-type slow blowing fuse wherein the basic fuse element material diffuses under heat into a metal adjacent to it so that it produces a new fuse alloy which in turn can blow and open the circuit under prolonged overloads. The outer compartments are filled with a granular arc-quenching material like sand for reasons to be explained, but the center compartment is not. Cylindrical end caps are secured over the ends of the housing to complete the assembly.
The fuse structure thus provides for immediate blowout under short circuit conditions where current many times the rated fuse current passing through the structure will melt one or both of the short circuit fuse elements. The arc-quenching material quickly quenches the arc which initially develops during short circuit conditions, so that the fuse immediately opens to protect the circuit involved and to avoid a dangerous explosion of the fuse housing.
On the other hand, slow blowing fuse elements are designed to pass moderate overload current surges in excess of the rated value for a brief period of time without blowing. Such a delay characteristic, for example, is particularly desirable in case of fuses designed for use with electric motors and other electrical loads having momentary high startup currents.
Current industrial safety standards demand that a slow blowing fuse element must blow at 135% of its "rated" value under prolonged load. Thus, a fuse rated at 15 amperes is required by these standards to blow at 20.25 amperes within one hour. These standards specify lesser periods of time at current ratings of 200% and 500% of rated current. The time delay is achieved by making the mass and resistance of the time delay element such that during passage of these various currents above rated values the temperature of the element will rise to its melting temperature with a desired delay and within the designated time period. The relevant factors to be taken into consideration in producing a desired delay are the resistance of the element, its length, its cross-sectional area, and the heat leakage rate from the time delay element to the surrounding environment. Thus, the various parameters of the slow blowing fuse element may be adjusted to provide the desired operating conditions of the fuse.
A further constraint posed by current safety standards is that the housing must not undergo explosive rupture from the overpressures generated attendant to short-circuit blowout, where short currents can reach hundreds of thousands of amperes. In the absence of special measures taken to minimize the effects of such overpressure, this typically mandates a large housing having very thick walls. On the other hand, a continuing design objective is the reduction in the size of fuse assemblies. To achieve non-rupturing fuse assemblies in housings of minimum dimension, a common recourse is to add the granular arc-quenching material surrounding the short circuit blowing elements. Such material may be made from a variety of substances. In addition to providing a large surface area facilitating free electron recombination in the expanding arc, thereby reducing the extinction voltage, certain filler materials are also believed to evolve electronegative gases with a similar ability to capture free electrons. The captured electron, now affixed to a much higher mass, is no longer effective in sustaining the discharge. Arc-quenching filler materials used for such purposes are well-known in the art, and include silica, boric acid, and fuller's earth.
As previously indicated, it has been customary to divide the fuse housing into separate compartments for the short circuit blowing and slow blowing fuse elements, so that the arc-quenching material is retained only in the outer compartments, leaving the slow blowing fuse element free of such materials.